1. Field of the Invention
The present invention relates to a circuit arrangement for offsetting the date heads of a data cylinder memory by a determinate amount from the so-called data cylinder mid-position (track offset), in particular using a fine regulating circuit and a coarse regulating circuit, and in particular to a circuit arrangement which comprises a track difference register, a digital/analog converter connected to the track difference register, a forward/reverse circuit which serves to form the sign, and a function generator, in which the amount of the desired offset is input, in digital form, into the track difference register, and is converted into an analog value by the digital/analog converter.
2. Description of the Prior Art
The positioning system for the write-read heads of a disc memory must fulfill two basic functions:
(1) To find new data tracks in as short a time as possible; and PA1 (2) To effect a precise position regulation of the heads on the data tracks during a reading process or a writing process.
Therefore, it is conventional to provide two regulating circuits which are intermeshed. A coarse regulating circuit is provided which contains a path-dependent speed regulation system of the positioner, and the path to be traveled to the new target track is stored, as a binary track difference (cylinder difference), in a difference register. The difference register is caused to count backwards with the cylinder pulses of the intersecting tracks. A digital/analog converter produces an analog value from the particular digital value. Depending upon the position of the target track, the analog value is then provided with a corresponding sign by way of a forward/reverse circuit, and with the aid of a function greater is distorted to form the theoretical speed signal. At the input of a summing amplifier, the theoretical and actual speed signals are then compared with one another, the difference between these signals is amplified, and a power amplifier is responsive to the signals to operate the positioning drive. A so-called "smoother" serves in a manner well known in the art to smooth the inconstant output function of the digital/analog converter.
A fine regulating system is responsible for the position of the data heads on the theoretical track. It is activated as soon as the difference register is reset to zero at the end of a positioning process. Here, the regulating value "path" is represented by the analog position error signal. In modern disc memories, this signal is derived from a special servo surface of the stack of discs. The position error signal is amplified in the summing amplifier and, by way of the power amplifier, causes the positioning drive to move in that direction in which the error signal becomes zero.
In addition to the two above-described basic functions, the position regulating device of a disc memory must also handle a series of subsidiary functions, including the function of "determinate offsetting of the data heads (track offset) by a few .mu.m from the data cylinder mid-position", e.g. in the case of data tracks which are recorded in offset fashion or for head adjustment purposes.
In known data-cylinder memories, the positioning system is offset from the data cylinder center in 63 different steps toward both sides, and the amount of the desired offset is entered into the difference register provided in the coarse regulating circuit. It is further known to convert this offset quantity, contained in the difference register, into an analog value by way of the analog digital converter which is connected to the difference register. The known positioning system for data cylinder memories then withdraws this analog value from the coarse regulating circuit of the positioning system and feeds the same by way of a plurality of switches to a separate device which serves to produce the relevant sign of the offset. From the latter, the offset signal, provided with the sign, finally passes to a summing point at which it is combined with the position error signal.
In order to derive the amount of the offset signal from the coarse regulating circuit, this known device requires a plurality of analog switches, and also requires a separate device to produce the relevant sign of the offset, which must initially be considered disadvantageous in respect of the expense of the components. Furthermore, these components form additional fault sources and affect the accuracy of positioning.